Soldering SMD's

[terry michaels]

I would like to request suggestions on the following problem.  Lately I
have been working on microwave circuitry, typically replacing a transistor
or other component.  This type of circuitry is built on an aluminum oxide
ceramic substrate with parts soldered to metallized traces on the ceramic. 
 The ceramic substrate conducts heat like crazy, so it is nearly impossible
to get enough heat into a small solder connection to melt the solder with a
small iron.  I can't use a large, high wattage iron to solder very small
SMD microwave components, it is physically too big.

These circuits were originally assembled by placing all of the components
on the pre-tinned substrate, and then heating the whole thing in an oven
until the solder melts.  This is not practical now that the substrate is
attached to a machined aluminum baseplate.  In some cases the entire
backside of the substrate is soldered to a metal baseplate.   I have heard
of people trying to heat it with the concentrated airflow from a heat gun
to remelt solder connections, but if the substrate is not heated evenly, it
might crack.

Any ideas?

Terry Michaels